text:
With the loss of a carrier, would American Navy aircraft consider landing on allied carriers in a major conflict?
Imagine a third world war or a major war involving significant use of carriers, something akin to a modern day variant of the battle of Midway involving carrier battles.
It isn't unthinkable that carriers could sink or be outright destroyed.
In a scenario where multiple allied nations are contributing carriers of different types, say for this particular battle, a couple carriers from the US, and a couple in total coming from the UK and France, the HMS Queen Elizabeth and Charles de Gaulle, respectively.
Being able to share resources throughout the battle (and war) appears to be a useful resource in and of itself.
In particular, what would the aircraft already launched do if their carrier is struck hard and is sinking? Aircraft are valuable assets, ejecting and ditching them in the sea I would imagine is the last option. Left with no other US carriers, no refueling, and no friendly airstrip nearby, is it part of the SOP (Standard Operating Procedure) to request landing on foreign friendly carriers? Even if it isn't part of the SOP, could it be done?
Moreover, should it be done?
My own reasoning:
I imagine that it could be done on at least the Charles de Gaulle.
On the Charles de Gaulle:
I assume the arrestor hook is compatible enough, although it wouldn't surprise me if there are implementation differences between nations on these systems that would render it useless. It would be interesting to know, if anyone knows for sure.
In any case, the Charles de Gaulle can (according to Wikipedia) take the E-2 Hawkeye, so I assume this is a strong indication it would be able to take an American E-2 Hawkeye as well. I don't assume they modify the arrestor hardware on the E-2 that the French use?
As for the F/A-18 and F-35C, I imagine they're using the same arrestor hardware as the E-2, hence they should also be able to land.
As for launching: Again the E-2 can be launched from the Charles de Gaulle, so it can probably launch a US Navy E-2 as well? The catapult link is the same? Same for the F/A-18 and F-35C? There's also the question of the power required to launch them, the calibration thereof, and the profile of how that power is delivered, perhaps this would be a challenge or somehow inflict undue stress to components.
On the Queen Elizabeth (and same with the HMS Prince of Wales):
The HMS Queen Elizabeth uses F-35B, but I believe these perform slow rolling landings.

Is it accurate that the Queen Elizabeth features no arrestor cabling at all? Does it have any nets that can capture aircraft? If not, I believe it would near impossible to safely land any non-compatible aircraft.
If they cannot even land, then launching is a moot question.

Although aircraft are expensive, pilots and carriers are much more so. It doesn't make a lot of sense to risk pilot and carrier for the sake of the plane with a landing that you aren't sure you can execute safely. With that said, I wouldn't be surprised if people had at least run the numbers about what's possible.

@Cadence Assuming that that arrestor gear works the same on the Charles de Gauller as a USN carrier, do you imagine the landing characteristics would be so different that it would be unsafe, in terms of risking the pilot and/or carrier?

This question has multiple issues. It's about naval strategy, not aviation per se, and it is opinion-based. I am voting to close.

Agree it is opinion based and involves naval strategy. There are some parts of it that are on-topic and factual, but they are extremely broad.  Perhaps it could be narrowed in focus to something like "could aircraft A from country B land on carrier C from country D".  I am voting to close.

P.S.  Most aircraft carriers just don't have enough deck or hangar space to add a full cycle of airplanes from another ship.  The real answer is probably no, simply because they wouldn't all fit.


text:
When should the term "pan-pan" be used instead of "Mayday"?
FAAO JO 7110.65, §10-1-1 states that "pan-pan" can be used to indicate a state of urgency:

a. An emergency can be either a Distress or an Urgency condition as defined in the "Pilot/Controller Glossary."
b. A pilot who encounters a Distress condition should declare an emergency by beginning the initial communication with the word "Mayday," preferably repeated three times. For an Urgency condition, the word "Pan-Pan" should be used in the same manner.

Under what conditions might a pilot use the phrase "pan-pan", versus "Mayday"?

I've been taught that in the UK a common PAN is when you find out you accidentally ended up in controlled airspace and tell 121.5  that you recognised the issue and request for assistance in getting out of there asap. This for gliders.

Oddly enough, the FARs and the AIM do not define these terms even though they use them frequently, but it is in the Pilot Controller Glossary (see below).  This is one of those areas that I've never really been given specific training on and I guess figured that I would "know it when I saw it".  The definitions kind of support that since they leave it to the discretion of the pilot:

EMERGENCY- A distress or an urgency condition.
DISTRESS (MAYDAY)- A condition of being threatened by serious and/or imminent
  danger and of requiring immediate assistance.
URGENCY (PAN-PAN)- A condition of being concerned about safety and of requiring
  timely but not immediate assistance; a potential distress condition.

Flight Safety released a report titled Use of Standard Phraseology by Flight Crews and Air Traffic Controllers Clarifies Aircraft Emergencies which includes a quote from an official in which she says that there is a misconception among some pilots about the difference between declaring mayday and pan-pan:

I have observed many cases where a mayday is given when pan-pan should
  be sufficient.  Many pilots do not realize that this distinction is
  ATC's way of prioritizing two or more aircraft with an emergency at
  the same time.

She also included some of the most important factors to consider when deciding to declare an emergency:

Is the aircraft in immediate danger?
Does the aircraft require immediate assistance?
Will the aircraft require priority handling during the approach or during any other phase of flight?
Will the aircraft need special assistance on the ground?
Does the crew need any assistance from other parties?

The most important thing though is not whether you declare exactly the right kind of emergency, but that you actually let them know when you need help.  Don't forget that you can also change your mind.  If you declared an Urgency condition and decide that you need more assistance/priority, you can always "upgrade" it to a Distress, and vice-versa.

It might be worth remembering that both these terms have a nautical origin. On the sea, pan-pan-pan (from the french en panne, meaning broken down) usually meant that your engine was out and that you were drifting. You were not in immediate danger but obviously, you could soon be hitting rocks or drifting into the path of other vessels with no way to alter your heading. May-day (from the french m'aidez meaning help me) usually meant that you were sinking.
These don't translate to aircraft so well since planes can't generally drift along aimlessly for very long. So probably the category for pan-pan is rather narrow.

The rule of thumb I was given when learning to fly (not in the US) was that mayday means "this aircraft is in immediate danger and I need assistance", whereas pan-pan means "I have an urgent safety issue but the aircraft is not in immediate danger".
Put another way, mayday always means that your own aircraft is in distress, but pan-pan is appropriate for reporting other aircraft in distress, ships or other vessels in distress, passenger emergencies aboard your own aircraft (someone has a heart attack and you need a diversion immediately) and so on.
But I think this is a rather grey area. For example, a few months ago a passenger jumped from a light aircraft in Florida. According to the definition I was taught, the pilot should have made a pan-pan call because there was no immediate danger to the aircraft. In fact, he made a mayday call, which might still have been very appropriate because if the pilot was so shaken (understandably) that he needed vectors or other immediate assistance in order to land safely then the aircraft was indeed in danger, thus justifying a mayday call.

if a passenger of mine had a heart attack, I would declare a "medical emergency" if there was any chance of saving them and I needed priority or assistance on the ground.

@Lnafziger The heart attack case was taught to me as a pan-pan scenario because the aircraft itself isn't in danger. But as you said these things aren't clearly defined and in the end it's all pilot's discretion anyway.

@Pondlife; the heart-attack is definitely a mayday scenario, a mayday is an immediate threat to the aircraft and/or its inhabitants, where I'd say a heart attack qualifies.

The door being open on the light aircraft could pose a danger to the aircraft... something inside could blow out or the door itself might come loose and strike the tail, which can cause damage and loss of control. Plus, buffeting from the altered airflow could pose a control problem. So a mayday was appropriate.

Mayday should be used if you or any passengers on board are in danger and need immediate assistance.
Pan-Pan is used when a safety problem exists, which might not necessarily be related to your plane (eg if you see another aircraft/vehicle in danger or if you for example notice a fire on ground).
Pan-Pan therefore informs potential rescuers (including emergency services and other craft in the area) that a safety problem exists whereas "Mayday" will call upon them to drop all other activities and immediately initiate a rescue attempt.

I know that in some parts of the world pan-pan-pan may be spoken after mayday-mayday-mayday to tell ATC and other pilots in the airspace that you have a problem, but want to solve it yourself (until further notice). ATC would then clear the immediate airspace to allow the aircraft in destress to maneuver however it needed to and work the problem without lots of other people and variables getting in the way. This actually happened on a Quantas a380 flight. After a stopover in Singapore, as it headed towards Australia, an engine basically exploded, which cut fuel lines, hydraulic lines, etc (basically, it was baddd). The pilots declared "pan-pan-pan" and eventually successfully landed back at Singapore.

I believe you're referring to Qantas Flight 32.

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text:
What can the Flight Attendant Panel do?
I've noticed that on some airlines (I may have seen it on SAS) the cabin crew had a small touchscreen at the front of the plane which they were using to select recorded audio messages etc, in both their language, and English.
Searching the internet, I found out it's called a Flight Attendant Panel — here are some photos I found:

So I gather they can control the lighting, and movies; but what else can these panels do?

I also found a FAP trainer, which says:

This virtual training environment generates a realistic FAP representation including OBRM, CAM and PRAM

What are OBRM, CAM and PRAM? What is being displayed above?

You posted this just to put those tags together.

I've seen this display too, at the rear kitchen area on a 777. One of flight attendants checked how full trash cans were in lavatories (aren't those trash cans high-tech), and also the seat numbers which pressed the call button.

I might be childish, but I lost it at "FAP trainer" :)

@Speldosa It was more the fact that they could play audio recordings in English and Norwegian.

@DannyBeckett Please try to bear in mind that, for more than 95% of the world's population, Delta is "a foreign airline". It just isn't useful to describe something as "foreign" in a forum where there are people from many countries. Since the nationality of the airline isn't at all relevant to your question, why mention it at all?

@DavidRicherby Please feel free to suggest a better wording! I was trying to convey that the cabin crew can select different language audio recordings.

@Speldosa - I suspect that most of the readers of this forum never heard of Major Hoople.

Here is an interesting presentation that discusses most of the communications systems on the A320 family (looks like the pictures you have and should apply in general). The CIDS (cabin intercommunication data system) starts about halfway through on page 21. The FAP is part of this system and can control:

Passenger reading lights
Passenger call buttons
Cabin lighting controls
Cabin system monitoring/testing
Passenger entertainment system
Passenger address (PA)
Automatic announcement and boarding music
Cabin/Service interphone and calls
EVAC signal
Emergency light
Door bottle pressure monitoring (either oxygen or escape slide?)
Door proximity sensors (doors closed properly)
Smoke detection
Water and waste tank quantities

The acronyms you listed are:

Cabin Assignment Module (CAM)

All the software for specific customer layouts and defined parameters in the FAP (same as OBRP).

On Board Replaceable Module (OBRM)

This is a generic term for any module that is easily replaceable. In this case it refers to the module that contains all the information used by the FAP (same as CAM).

Prerecorded Announcement and Music (PRAM)

A good list of Airbus acronyms including these can be found here.

seem to be russian and there doesn't seem to be a .pdf anywhere but still thank you.

The OBRM, PRAM, CAM are three memory cards, which hold aircraft related data and softwares and plugged into the FAP at botton. (You can see three slots).
The page shown in the picture is cabin status page, which is showing current status of aircraft namely Music, Lights, Doors, temeperature and lavatories.

In simpler times the panel didn't do much at all.

this should have been a comment.

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Why are some US STAR fixes/waypoints included in FAA's legacy files but not in FAA's published STAR procedures?
I have noticed that some US STAR procedures include (slightly) different number of waypoints, depending on which source material is consulted. Take for example the CAMRN4 STAR and its corresponding description from the legacy files provided by FAA. In the legacy files, the reporting point GAMBY is mentioned between KARRS and CAMRN, whereas in the published information it is omitted.

S0095  NW N3905438W07448012SIE     SIE.CAMRN4   CAMRN FOUR
S0095  R  N3924521W07427171BOTON K6   S0095  R  N3934583W07416141HOGGS
K6
S0095  R  N3940336W07410055PANZE K6
S0095  R  N3950271W07359096KARRS K6
S0095  R  N4000222W07352083GAMBY K6
S0095  R  N4001023W07351398CAMRN K6
S0095  AA N4043436W07324483FRG
S0095  AA N4038237W07346433JFK

Is there a reason for this or have I misunderstood something?

your first link is 404 broken. edit second link also leads to "not found"

Revisions are made to improve traffic flow, adjust for infrastructure(eg local airport change in number of flights or nav beacon decommissioned), simplify procedures for pilots and ATC, or address some other routing conflict or safety issue.
This is why IFR charts and terminal procedures are only valid for two months.
For example there is a victor airway near here that had its MEA raised from 3000 to 5000 because it was discovered that at the lower altitudes something was causing the VOR signal to bend significantly and planes were getting dangerously close to a mountain, I think they may have even had a couple of crashes.

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According to ICAO, which rules are applicable to SBAS Non-Precision Approach Procedures?
I am studying the Satellite Based Augmentation System (SBAS) from PANS OPS manual 8168, Volume II, sixth edition, and I have a problem concerning the criteria of SBAS-NPA.
In Part III, Section 3, Chapter 5 called "SBAS NON-PRECISION APPROACH, APPROACH
WITH VERTICAL GUIDANCE AND PRECISION APPROACH
CAT I PROCEDURES", it states:

5.1.1 Procedure construction This chapter describes the SBAS criteria for the NPA, APV 1 and PA CAT I procedure segment, which are specific to
the performance of SBAS systems. Throughout this Chapter SBAS OAS refers to both SBAS APV OAS and SBAS
CAT I OAS. The APV or CAT I segment includes the final approach, and the initial and the intermediate phases of the
missed approach segment. The other phases of flight are generic in character and are presented in Part III, Section 3,
Chapter 1 and Chapter 2.

However, a few chapters before and, specifically, in Chapter 3 called "NON-PRECISION APPROACH PROCEDURES", it describes the rules concerning the Non-Precision approaches in general.

3.1.1.1 This chapter describes the non-precision approach criteria for RNP APCH procedures.

The problem is that it's not clear to me which criteria to follow. Do the rules of the general Non-Precision approaches also apply to SBAS Non-Precision procedures, or the latter follows its own distinct rules covered solely in Chapter 5?
Another issue that troubles me is that the topics discussed in this manual are scattered through its sections, without any easy way to track the desired information. For example, in Volume I, it states:

1.2.4.2 Straight-in approach. Wherever possible, a straight-in approach will be specified which is aligned with
the runway centre line. The pilot should be aware that for non-precision approaches, a straight-in approach is
considered acceptable if the angle between the final approach track and the runway centre line is 30° or less

On the other hand, in the aformentioned Chapter 5 of Volume II, Part III, where it discusses SBAS, it has a special paragraph discussing the use of SBAS approach with offset alignment.

5.7.1.1 The final approach track shall intersect the runway extended centre line:
a) at an angle not exceeding 5°; and
b) at a point where the nominal glide path reaches a height called intercept height of at least 55 m (180 ft) above
threshold.

This becomes really confusing, since I can not comprehend which rules apply exclusively for SBAS NPA and which rules are supposed to be followed in every case of non-approach procedure.
I would be grateful if someone could help me clear this out and read the manual the way it is intended by their authors.


text:
Would turbulence created by two jumbos less than 100ft apart cause both of them to crash?
In a scene from Spider-Man 2 (2014), which you can see in this video, two jumbos almost collided. At the last possible moment, the pilots were able to save their planes, but both planes came to within 100 feet apart from each other (I guessed the distance). Both planes survived.
I wonder in real life with real science and no Hollywood involvement, could any plane survive this close encounter with another jet?

tcas would never let them come so close.

@anshabhi TCAS is not magic. Aircraft equipped with TCAS have come within 100 feet, and closer (metal-on-metal close in some cases).

@voretaq7 ,  isn't turbulence causing serious problem for jets that close?

For what it's worth, it's not two jumbos. It's a 747 and a narrow-body jet.

The probability of two jets surviving such a close encounter are extremely remote.
Setting aside the fact that it is difficult to imagine two aircraft flying in the same altitude in opposite directions (commercial aircraft are separated by 3 miles laterally and 1000 ft. vertically) ,if the scenario unfolds as in the movie and the horizontal separation between the aircraft on approach is as shown, the pilots would not have time to react and take the necessary evasive actions.
Assuming that the jets are cruising towards each other at 900 kmph, the combined speed is around 1800 kmph, or half a kilometer per second.
If the TCAS is not working and the aircraft come within five km (the distance in the video is considerably lower, less than a km) of each other, the pilots have less than 10 seconds to get out of each other's way.
The civil aircrafts are not designed for such rapid roll rates. The inertia of the aircraft (especially the 747) will limit it.
Another thing to note is that the modern flight control systems will not allow airline pilots to carry the maneuvers shown in the flight. The pilot can override the Flight control Computer.However,in this case, they would not have time to react.
If by some miracle the aircrafts cross each other, they will fly right into the other aircraft's wake turbulence, which will have disastrous consequences, especially for the smaller aircraft.

Sources: airpower.maxwell.af
In case of small aircraft flying behind a heavy jet, FAA regulations require a minimum separation of 5 miles, lesser than which the effect of the wake turbulence (wingtip vortices) can have serious consequences.

Source: Aeronautical Information Manual
FAA conducted a test on the effects of wake turbulence by flying a 737 behind a 727, which shows the loss of control associated with flying in the wake of another aircraft.
Once (if) the aircraft pass each other, they should be brought to level flight as quickly as possible (whether it is possible or not is another question) to prevent stall. In this critical juncture, if the aircraft wings encounter wake turbulence, the control of the aircraft will be loss, resulting in stall.
Finally, in case two aircraft do approach each other in the same altitude in opposite directions, there are laid down procedures to follow. None of them involve the stunt shown in the movie.
When two aircraft are flying head on, each must move to their right.

Source: planefinder.net
If two aircraft are at the same height and on converging courses, the aircraft with the other on its starboard side has to give way.

Source: planefinder.net

Great explanation. I understand it now.

In a close collision situation (where the pilots have not seen each other until they are nearly colliding, it may not be possible to follow these rules, because banking takes time and the upturned wing might hit the other aircraft. Pilots in this situation might consider one will go up the other will go down, although there are no rules for this, and because going down is easier (and faster) both might decide to go down. This is why I would always try to fly at a different altitude to other traffic in the close vicinity where possible.

"commercial aircraft are separated by 3 miles laterally and 1000 ft. vertically" - What's the logic behind this? and is 1000ft safe, because we hear frequently that planes suddenly lose altitude due to updraft / downdraft etc..

Wake turbulence is a serious problem if flying too close to another aircraft, for example if a light aircraft follows a larger aircraft, the sheer amount of air and wind speeds involved could cause a loss of control and possible subsequent damage to the aircraft if not rated for aerobatics (or if close to the ground). You have to stay quite a long way behind a larger aircraft when approaching to land to be sure of not encountering it.
However if encountering wake turbulence from an aircraft of a similar size, while you would most definitely feel it, I don't think it would cause a loss of control.
Even so, when flying in formation aircraft stay away from the others wake turbulence because although I don't think it would cause them to crash it would be uncomfortable and the formation wouldn't look smooth if at an airshow for example.
To answer the other part of your question - would it affect the aircraft in front? Only if the aircraft behind lost control and hit it, as was thought to happen to Joe Walker who was flying a F104 and hit the aircraft he was chasing causing both to go down

, it was suspected that the F104 was caught wake turbulence from the large aircraft.

This answer doesn't have very much to do with the question. The situation in the video does not involve one plane flying behind another; rather, it shows then narrowly avoiding a head-on collision.

It wasn't head on, the F104 was flying on the wing of the XB-70 in the same direction and the F104 got caught in the XB-70's wingtip vortice (part of the wake turbulence) and got carried along by that vortice until it hit the top of the XB-70. Sorry you don't think it was relevant, I was only trying to help, I used it to demonstrate that while the F104 was washed around like being inside a washing machine, the larger aircraft didn't move at all, until later in the accident sequence when control was lost due to having no fins or rudders.

I'm sorry I haven't seen the film. If it was head on in the film, then wake turbulence would have no affect. Whether the planes survived or not would simply depend on the amount of G's pulled to avoid each other.As a pilot I took the word Turbulence in the original question to mean wake turbulence, because there isn't really any other kind created by an aircraft that could affect another aircraft.

Sorry, I was unclear. I meant that the video in the question is a head-on situation.


text:
Why don't we have helicopter airlines?
Just a curious thought. Why isn't there any helicopter airline? Airlines that operate large helicopters that can transport 10 or 20 people a short distance?

If it didn't look like I was just trying to be flippant, I would answer "because we have cars".  Define what you mean by short distances.

What do you mean by 'airline'?  There are obviously many helicopter charters.

You are invited to investigate the Off Shore industry's use of helicopters to take people to and from oil platforms.  CHC, Bristow, PHI are but three such companies.  Those are indeed "helicopter airlines."

There is one in NY that operates as a kind of charter (pushing the FAR 135 Rules a bit).

We do. Air Greenland is a good example.

There is Uber chopper in Dubai and various other cities

Donald Trump operated the Trump Shuttle for a while that was both airplanes and helicopters.  https://en.wikipedia.org/wiki/Trump_Shuttle

I'll just leave a link to Copterline here. They ran regular daily routes between Helsinki and Tallinn, priced for business travellers (200 eur per seat). Nowadays they seem to serve oil rigs only. (The Helsinki-Tallinn route was 18 minutes per trip; for comparison, the (very popular) ship routes take 1,5-3 hours IIRC)

@Adam -- "Trump Shuttle never turned a profit. In September 1990 the loans were defaulted and ownership of the airline passed to its creditor banks."

Helicopters are very slow, very short range, and very expensive.  It's hard to sell a slow, expensive, short range flight.

@J... slow and expensive are both relative. Many serve routes where the alternative is even slower and/or more expensive. Alternatives like boats to oil rigs.

@jwenting  Naturally.  I don't think, however, that OP is talking about airline routes to oil rigs.  The idea is pretty clearly helicopters as an alternative to taxis, buses, trams, or trains for short hops.  Surely there are places where helicopters are a good fit - and not just for rare oddities like routes to oil rigs - but the areas where they are economically and practically superior alternative are few, though they do certainly exist.  It's a niche mode of transport rather than a general purpose one.

Sure, one of the most famous ones is Air Monaco. I lived in Aberdeen, Scotland for some time and (it's a specialized thing for the oil rigs but, ) it is said to be the airport in the world most busy with scheduled heli flights from heli airlines.

Why? because economics.

They do in fact exist, though they often use smaller helicopters. A Google search on the term "heli taxi" yields thousands of results, and while a lot are probably irrelevant, the first few pages give hundreds of operators around the world, ranging from companies ferrying passengers between airports and major cities to companies servicing oil platforms at sea, islands too small to have airfields, etc.

Keep in mind that an airline is a company, companies usually try to be profitable, and helicopters just aren't cheap. Pretty much any other mode of transport is cheaper. You therefore tend to see helicopter services where there are no decent alternatives

In addition to the cost, I believe the accident rate is higher. Oil rig personnel are required to do the (arduous and unpleasant) training for evacuating from a helicopter that's ditched at sea.

@pjc50 As somebody who has taken helicopters out to oil rigs myself, I can tell you that it is not a requirement. It is usually enforced by the helicopter operator, but not always. The biggest certification programs are SafeGulf and HUET. I've taken a helicopter without either training (although I didn't enjoy it, it was an emergency and didn't have time for training).

@RonBeyer our guys speak of the training experience in hushed tones. I'm still trying to get myself on it to see what the fuss is about :D

@MSalters: the last time I took a helicopter in Nice (from the airport to a hotel in Monaco), the fare was equivalent to a taxi on the same route. It was way faster, though.

Air Greenland is a good example of a company that uses helicopters in scheduled air traffic. Unlike many of the other examples mentioned here, a large part of Air Greenland's helicopter operations take place as scheduled traffic, and not charter flights. They operate the following:
2x Sikorsky S-61N with 19 seats
9x Eurocopter AS 350 with 5 seats
8x Bell 212 with 8 seats
to and from multiple small cities and towns on the coast of Greenland.

The Air Greenland helicopter serivces mesh with their Dash-8s and A330 routes, so this is a great example of an airline using helicopters as part of a full scheduled service, not just a niche.

The other answers demonstrate that there are several helicopter airlines, but I don't think that really answers the question.
Why aren't there more? Simple economics. Compared to other kinds of powered aircraft, helicopters are small, slow and expensive to run. They do have the advantage that a helipad takes up much less space than a runway which means you could fly your helicopters right into the middle of the city, rather than to large airports in the suburbs or beyond. However, helicopters are also very noisy, so that would be very unpopular with everybody else who used that part of the city. This means that your heliport actually has to be out in the suburbs anyway, and you've just lost your only advantage.
All of this makes it very difficult to be a profitable helicopter airline. All of the examples in the other answers are cases where special circumstances mean that helicopters can still work. Taking people to oilrigs by air is much faster than by ship, but you can't land a fixed-wing plane on an oil rig. Small, widely spaced communities might not be accessible by road and might not have the resources to maintain an airport, but might be willing to pay enough to support a helicopter service when it's their only option. If you just want to transport "10–20 people a short distance", a bus is going to be almost as fast, much more comfortable and orders of magnitude cheaper in almost all non-special circumstances.

Great answer. In the case of the Alaska helicopter service I mentioned in my own answer, it is subsidized by the government as part of a program to keep remote communities connected to the rest of the country. If Erickson had to rely only on passenger fares to run the service, it would probably not be economically viable. It is very similar to the current situation of Amtrak rail in the USA, which the government keeps running at a loss because it considers it to be an essential service that shouldn't close due to market pressure.

We do: Helijet International.
Helijet International is a helicopter airline and charter service based in Vancouver, British Columbia, Canada.
They operate regularly scheduled flights between:

Vancouver Harbour Heliport and Victoria Harbour Heliport
Vancouver Harbour Heliport and Nanaimo Harbour Heliport (Monday-Friday)
Vancouver International Airport and Victoria Harbour Heliport
(Monday-Friday, limited flights)

I'm sure there must be other services where the geography and economy can support it.

According to their website, Helijet offers a 35-minute flight from Nanaimo Harbour (wherever that is) to Vancouver Airport for a very reasonable $109.  I would do it once just for the experience.

That's almost cheaper than BC Ferries!

This service is feasible because Vancouver airport is 10 km from downtown Vancouver, and Victoria airport is 25 km from downtown Victoria. Add to that the time savings of expedited security clearance, and it is much faster than any of the alternatives.

There are helicopter airlines.
Definition: airline is a company that transports people by air and in so doing makes a profit (or tries to).
PHI, Bristow, and CHC are three of numerous companies who transport people to and from oil platforms as passengers, for a profit.  (At least, that's their aim).

I am reluctant to accept this definition of airline as it would encompass all unscheduled, on-demand part 135 service, including medevac air ambulance service. While the part 119 certificate holder of an air-ambulance may be considered an airline for some purposes, I should think their part 135 operation would not be.

Such companies, such as the one for whom I work, do not publish a schedule of flights, and operate on demand. As a pilot I do not have a flight schedule, only a duty schedule.

@JonathanWalters Possible response: "demand pull" is a more efficient way to run a business where the  margins are what they are in the helicopter off shore transport business.  You serve your customer base when they need to move people, and you schedule/adapt to meet what the customer needs, rather than move a plane and tell this customer base "hope your people are ready to move."  The difference passenger in volume (rotary wing versus off shore) is orders of magnitude so economies of scale don't work the same.

I suppose I should have left the comment at the question, not an answer. I've done so now.

I think the more commonly acceptable definition of an airline is: "an organization providing a regular public service of air transport on one or more routes.". I don't think any over the organizations you mentioned meets the "regular public service" part.

Erickson Aviation operates a regular helicopter service from Nome, Alaska to Little Diomede Island, Alaska. Little Diomede sometimes makes it onto lists of the most remote towns in the USA and is more or less a small rock in the middle of the Bering Strait with a fishing village on it.

The helicopter option is popular with Hong Kong's high rollers, looking to hit the tables in Macau.
http://www.skyshuttlehk.com/

In the UK, British International Helicopters (formerly British Airways Helicopters) is a helicopter airline, although they do not currently fly any regular public routes.
BIH fly for offshore (e.g. North Sea oil) and defence industries.  They also offer training and sightseeing tours from London (Redhill) and Coventry.
BIH operated a regular Sikorsky S-61 service from Penzance Heliport to St Mary's Airport and Tresco Heliport on the Isles of Scilly from 1964 until 2012, when Penzance Heliport was sold to the supermarket Tesco.  There are plans to reinstate the service if a suitable new site can be found.
BIH jointly operated the Airlink shuttle service between Gatwick and Heathrow between 1978 and 1986.  The service ended because its licence was revoked after the M25 motorway had been completed.

New York Airways used to operate a helicopter taxi service until they closed in 1979. They operated a number of helicopters, including the tandem rotor BV107-II. If you ever watch the Clint Eastwood film Coogan's Bluff (1968), you'll see at least two of them operating off the Pan-Am building.
New York Airways

I think explaining why they closed would make this much more appropriate to the question.


text:
Can civilian aircraft fly through or land in restricted airspace in an emergency?
This is a super hypothetical question of mine. I was wondering if civilian pilots who happened to be faced with a severe emergency could fly through or possibly request to land in restricted airspace? Mainly I am referring to Area 51?

Physically, short of shooting you down, they couldn't stop you landing anywhere. If you could get permission before hand is another matter. The interrogation after may not be pleasant either.

In an emergency, the captain can do whatever they deem to be appropriate, including flying or landing in restricted airspace following the basic rule of "aviate, navigate, communicate".  It must be the only option available and the captain would need to be ready to explain why in the enquiry.

It has been reported, sort of.

FAR 91.3 states that any pilot in command may deviate from any regulation or rules to the extent needed to deal with the emergency.  That includes entering restricted or prohibited airspace.
In the example that you suggested, yes you could do so and land at the Groom Lake flight test facility at KXTA, provided you could reach their controllers or contact Nellis Approach Control declaring an emergency.  I will say that if you do enter R-4808 N and land there, don't expect them to roll out the welcome mat.  You will be immediately met by USAF Military Police who will arrest you, most likely at gunpoint, and spend several hours, if not days, there or outside the compound being extensively questioned by Air Force Security personnel, the FBI and Homeland Security.  Your aircraft will be impounded and inspected, possible indefinitely, and search warrants issued for your property.  If they find out that you faked an emergency to enter that compound, you are in very serious trouble and face a variety of federal charges including trespass upon a classified installation and whatever else the DoJ can throw at you.
In addition, you would need a very, very good reason to even be anywhere near Area 51, as it is in the middle of a military reservation geographically the size of the nation of Switzerland in the middle of south central Nevada.  There are no published routes over the airspace and all commercial and civilian traffic is kept far, far away.  Details about the airfield are not published on government aeronautical charts or regional A/FDs.  See the VFR sectionals and enroute charts below.

While Area 51 is a pretty extreme example, people have had emergencies and landed in military or other restricted facilities.  The usual response from the authorities is to detain and question you to determine what happened and inspect the aircraft to verify your story.  If it's a legitimate emergency, no harm is done and they will release you and the aircraft; the NTSB will file and incident report and that's it.

If you have planned a flight where your best diversion point is Groom Lake then you would probably loose your ticket on that basis.

@GdD UA232 springs to mind.  You might not plan it as a diversion but fate might make it necessary. It's not inconceivable that lateral navigation, or lack of it, permits no planned alternative.

I understand the point you are making @Simon, and I agree that circumstances can put one in a position never anticipated. I am saying that you shouldn't put yourself in a position where your only option in a more "routine" emergency would be such a sensitive area.

Presmably there's a principle here that the right (indeed, obligation) to deviate from aviation regulations and rules doesn't necessarily grant complete immunity from prosecution on all other grounds. Just means it won't be the FAA prosecuting. So if you can handle your emergency without committing any other offence then you're good to go.

KTNX in the northwest corner of the Nellis range -- Tonopah Test Range, former home of the F-117's -- would be a better case of landing in the restricted area as a result of something gravely serious (in-flight fire would do it).  It would even have the added detail of being a more suitable airport than, say KTPH (civillian Tonopah) just outside the restricted area -- at least for a reasonably large aircraft.

@RalphJ -- yeah, KTPH's only straight-in IAP (RNAV RWY 15) is 1) CAT D NA and 2) not usable by westbound traffic on V244 at WURIL.  There is a circling VOR/DME approach, but many airlines don't have circling approaches in actual IMC in their OpSpecs...so KTNX with its ILS and longer runway looks mighty attractive at that point!

Being an air traffic controller I know this situation is not as simple as that.
Given emergency category pilot can decide to use restricted area or land a military runway. But in the mean time pilot needs to inform ATCo's so they can coordinate with civil and military aviation authorities to keep flight from scramble jets, locked in air defense etc.
If pilot cannot communicate with ATCo (so he/she will need to squawk 7700) ATCo should guess what pilot is going to do and inform authorities.
Given situations above yes, an emergency plane can fly through restiricted areas etc. But if there is any other way out pilot shouldn't choose to use restricted area like Area 51. If it comes out he/she had other options but still used Area 51, that will be a big headache for the pilot. (and ATCo needs to advice pilot that there might be other ways..)

I watched an AOPA video about a pilot in IMC having trouble finding a place to land and was running out of gas. He asked the ATC at Dover AFB if he could land there but the ATC told him no, unless it was an emergency. Unfortunately he was reticent to declare so he continued on and fueled out in IMC. He didn't make it.

@TomMcW: I guess that would be Final Approach from Accident Case Studies - sobering stuff!

@RedGrittyBrick That's the one. I watch it and keep wishing this time he would declare an emergency. He'd face some hard questions, but he'd live to tell about it

I belive eclaring emergency is a harsh option for pilots.
For example; because of heavy traffic around airport we had to delay some planes. After 30 min circling a pilot said he is running out of fuel and may declare emergency if we do not land him in 10 minutes. We asked if he is declaring emergency and he said he is not, but he will in 10 minutes. We said ok call back in 10 minutes. After 30 minutes he landed but never declared emergency.
Was he running out of gas? If so why didn't decleared emergency fuel?
If he wasn't running out of gas, why he tried to negotiage an earlier landing?

@TomMcW what i wanted to tell above is you can't trust pilots all the time. Some doesn't declare emergency because of paperwork but wants to treated like an emergency. Too complicated.

@alkinkasap -- in the USA, there is no mandatory paperwork involved with declaring an emergency.

It's a principle handed down to aviation from the traditional laws of the sea: Any port in a storm.
In fact, if a pilot declares an emergency and asks for vectors to the nearest suitable landing environment, ATC might well direct him to an airfield that otherwise would be denied to him.

First, Groom Lake is a lousy example, because it's located roughly smack in the middle of the always-busy Nellis Range Complex, which keeps nonparticipating aircraft far, far away.  (The only emergency diverts it ever sees are Nellis Range participants who have to knock-it-off in a big hurry, I'm sure.)
OTOH, in a general sense of the question, the answer is "yes, if necessary for safety of flight, a civilian emergency can land at an airfield in restricted airspace."  A good example of this would be if you were puttering along the Western portion of the Eglin FAR 93 E/W corridor and something terrible happened -- making an emergency landing at Hurlburt Field would definitely be preferable to an off-airport landing or ditching.  Obviously, you'd be telling Eglin Approach about your predicament since you have an advisory service from them anyway, and the controller would simply tell the Eglin Range folks to broadcast a hold or knock-it-off to whoever's working in R-2915B until you're safely on the ground.

Yes they can if there's no other airport near the airliners reach. Lets say the plane has 2 engines fail on one wing that is counted as an emergency so they will need to land were ever it's safe to land.
The Hudson river landing is one of the best landings done by a pilot with 2 engine failure on both wings.

"done by a pilot"... by a crew?

The Hudson river is not exactly a great example of a restricted airspace.

The Hudson river is not a great example of a runway, either.

@Criggie - don't tell that to Sully!

@Criggie Sully would never call the Hudson River a runway. He's a pilot.


text:
How do propeller-driven, fixed-wing UAVs counter negative torque from engine rotation?
What cancels the rolling moment produced from the rotation of the engine?

I don't understand why UAVs would be any different in design from ordinary propeller aircraft here. Aerodynamically, you have all the same considerations.

In exactly the same way as any conventional, single-prop aircraft does
Which is to say, by using stabilizer and aileron/rudder trim, and the natural resistance that wings/stabilizers give.
The wings/stabilizers will naturally resist rolling forces to some extent, particularly at flight speed. At lower speeds (eg when taking off or landing) the control surfaces can counter the force quite easily - a little opposite rudder and aileron is all that's required.

People underestimate how MUCH wings counter torque just by being there (no asymmetric modifications, no aileron adjustments etc). Try this. Get yourself a rubber powered toy plane. Does not have to be anything fancy. Wind it up without the wings on and let go. See how fast the body spins? Now put the wings on (no need to assemble the tail etc, this is a simple experiment) and let go. Notice that the plane would not make even one full roll before hitting the ground. In reality the pilot subconsciously adjust banking angle. The adjustment is so small that you won't really notice it.

The design considerations for UAVs are the same as that of other aircraft.
The torque applied by the motor (or engine) to the propeller causes an equal and opposite torque applied by the propeller to the UAV body. This torque, $T$ is opposite to the propeller rotation and is proportional to the square of propeller angular velocity ${\Omega}$, given by,
$T = -K_{T_{P}} * {\Omega}^{2}$,
with $K_{T_{P}}$ being an experimental constant. The effects of this torque are relatively small for small UAVs. In case of large UAVs, these effects are nullified by the same control inputs(aileron) as in other aircraft.
In case of multi-propeller UAVs, it may be desirable to use counter-rotating propellers so that the net torque generated by the propellers as they rotate is canceled to zero. Using of contra rotating propellers is also an option.

Source: www.unmannedtech.co.uk
A unique case for UAVs is the quadcopter, where two propellers (rotors) spin clockwise and two spin counter-clockwise, canceling the net torque on the aircraft.

Quadcopters, hexacopters, octacopters... but yes.

How about quintacoptors and septacoptors?  :)

As noted above, whether piloted or UAV a/c, it makes no difference.  An aircraft design counters engine torque in one of several ways.
ASYMMETRICAL WING TWIST - In many light GA aircraft, the angle of incidence of the wing opposite the descending side of the propeller arc is greater than the angle of incidence for the wing on the descending side of the propeller arc.  This creates an imbalance in lift between the two wings and helps to counteract the propeller torque.  However the amount of counter torque is fixed and may have to be augmented by control inputs at higher speeds.
AILERON TRIM - Some aircraft, in particular large twins which do not have counter-rotating propellers have trim tabs on the ailerons which allows the pilots to counteract roll torque about the longitudinal axis of the aircraft.  This is more advantageous than the asymmetrical wing twist described above in that the roll trim can be adjusted for takeoff/climb/cruise and landing airspeeds and engine power settings.
COUNTER-ROTATING ENGINES/PROPELLERS - Many twins have engines which are geared to rotate their propellers in opposite directions such that each engine's torque cancels out the torque of the other.  This has an additional benefit of reducing thrust asymmetry during an engine failure by eliminating a critical engine from the design.
REACTIVE COUNTER TORQUE:  In some turboprops, mainly the PT-6 and other reverse flow, free turbine designs, the exhaust stubs are designed to divert exhaust gases from the power turbine in opposing directions offset from the centerline of the powers haft, creating a reactive torque which counters the torque of the propeller.  This design concept is popular with turbine conversions of existing reciprocating engine powered aircraft eg. the PA-46 JetProp conversion, etc., where the original airframe was design around a lower power engine which produced less torque.
Another design, virtually unheard of in aviation today, is to turn the propeller by means of a stream of high pressure gas emanating tangentially from the propeller tips.  This eliminates any kind of torque effect about the airframe from the turning of the propeller but is a superfluous design in an age of jet aircraft.
CONTRA-ROTATING PROPELLERS - This option was explored during WWII on military fighters and is currently used today on several Unlimited-Class Air Racers eg Precious Metal, the old Red Baron RB-51 prior to its destruction in a crash.  It consists of two sets of propellers mounted to a gearbox which rotate in opposite directions.  The design has several advantages in that it not only eliminates engine torque, but also the effects of P-Factor, gyroscopic precession, and propwash on the fuselage and empennage of the aircraft.  The downside of the Contra Rotating Propellor is the expense and the excess weight of the gearbox and extra propellor.

authors: Юрий Матюшин


text:
In an emergency, can an airliner land at Pyongyang or another North Korean airport?
If a flight was, say, en route to Seoul, but could only make it to the closest available airstrip and that was Pyongyang, would that be allowed? What would they be met with on the ground?

I would be more worried about being met by AAA before arriving at the ground.

You can get AAA coverage in North Korea @MartinJames? Does the 50 miles free towing cover 747s?  ;)

In an emergency, a pilot can do anything he thinks has the greatest chance of ensuring everyone aboard gets back off safe and sound. A helicopter pilot in DC experiencing an engine failure can attempt an autorotation landing on the roof of the White House if that will get him and his passengers back on the ground alive and well. However, getting off the plane alive is just the first hurdle.

The situation in the question is extremely unlikely. For Pyongyang to be closer than Seoul, you'd essentially have to be coming from the north, in which case your plan would have to have been to overfly North Korea, which airliners don't do. From the west, flights come in over the Yellow Sea and Seoul will be closer; from anywhere south of Seoul, you'd have to fly right past Seoul to get to Pyongyang; from the north-east (e.g., great circle from California), by the time you've passed Vladivostok, Seoul and Pyongyang are about the same distance.

I can see at least three questions here and only one of them is fully aviation-related. First, can an airliner legally divert to Pyongyang in an emergency, under the laws of its own country? For US airlines, the answer is yes. 14 CFR 91 SFAR 79 bans flights over North Korea, but does allow an exception in an emergency:

2. Flight Prohibition. Except as provided in paragraphs 3 and 4 of this SFAR, no person described in paragraph 1 may conduct flight
  operations through the Pyongyang FIR west of 132 degrees east
  longitude.
[...]
4. Emergency situations. In an emergency that requires immediate decision and action for the safety of the flight, the pilot in command
  on an aircraft may deviate from this SFAR to the extent required by
  that emergency.

Second, will North Korea allow the airliner to land? This is a political question, not an aviation one. Either they say yes or they say no. If they say no to an airliner in distress, how will they enforce that? Are they willing to shoot it down rather than let it land? Would that decision depend on the country of the airliner? Can they even make that decision within minutes, or is it already made and they simply initiate a defined procedure? The pilot is probably just going to try to land anyway; assuming the alternative is a crash, he would have little to lose.
Finally, what will the 'reception committee' on the ground look like? No idea, this is a political/military question.
But practically speaking, this situation should never arise anyway (ignoring movie plot stuff like hacked navigation systems). If your country isn't friendly or at least neutral to North Korea, your airlines simply won't plan flights where the only emergency landing possible is in Pyongyang, and for US carriers specifically it would be illegal anyway.

In an emergency that requires landing as soon as possible an aircraft can (and should) land at the nearest suitable airport, laws and international incidents be damned. If your 747 is on fire then being on the ground at an airport in North Korea is preferable to exploding in the air or winding up in the ocean or on the ground somewhere that's not an airport.
As Pondlife noted the regulations prohibiting flight over North Korea contain a specific provision lifting the restriction in the event of an emergency, in addition to FAR 91.3's provision that:

In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.

Everything else isn't an aviation problem: This is why countries have diplomats (and, if necessary, their armed forces), however as North Korea (or more properly "The Democratic People’s Republic of Korea") has submitted a "Notice of Adherence to the Chicago Convention" they're bound by its requirements, specifically Article 25 which states in part:

Each contracting State undertakes to provide such measures of assistance to aircraft in distress in its territory as it may find practicable, and to permit, subject to control by its own authorities, the owners of the aircraft or authorities of the State in which the aircraft is registered to provide such measures of assistance as may be necessitated by the circumstances.

There are very few circumstances where allowing an aircraft in distress to land would be "not practicable", though nothing requires them to let the passengers/crew off the aircraft, provide other support services, or make it "convenient" for the passengers & crew to get back home.

If I were an American pilot and it was a choice between landing in Pyongyang or ditching off the coast of Japan or South Korea, I'd choose the sea. I think the idea that North Korea is "bound" by the Chicago Convention is pretty risible.

I would imagine compassionately allowing an "enemy" plane to land and heroically rescuing its passengers would be a welcome international PR opportunity for the Great Leader.

I think you should treat a North Korean airport as any other one. Since you're protected under international law, they can't do anything, especially if it's an emergency landing. Plus, I don't think their government would want to do something. They would probably lend a hand and help out, for their reputation. At least, if I was Kim Jiang-Un, I would do that.

Do you have sources other than just your opinion?

North Korea doesn't have the greatest record of respecting international law or appearing soft in their reputation.


text:
Why couldn’t Boeing just use the stab trim system to do MCAS job?
If I’m incorrect in the question, please correct me. From what I’ve read and gathered the stab trim system essentially did the same before hand? It relies on AoA? And if so, did the computer for that take information from one AoA at a time like MCAS did? I’m pretty sure I’m wrong anyway, this is super confusing for me, but that’s my understanding as of now, so basically Boeing needed the trim system to make a further command than the previous system, correct?

Which previous system are you referring to?

Oh, I thought I was referring to the stab trim system? I thought the AOA sensors on the Next generation would be feeding the info to that?

No, the 737 NG stabilizer trim system does not feedback from AOA. My source of information comes from this LinkedIn Slide Share and corroborated by this online FCOM.
There are several ways through which the stabilizer can be commanded:

Manually via the trim switches.
Manually via the trim wheels.
Automatically via the Speed Trim System (STS). A helpful explanation of the STS can be found in this answer. STS feeds back from stabilizer position, thrust lever position, airspeed and vertical speed.
Automatically via the Autopilot trim offset. This is only applicable with the relevant autopilot mode engaged.

The Mach Trim function also modifies the trim characteristics. However, it does not actuate the stabilizer and repositions the elevator centering and feel unit instead, which backdrives the column.
MCAS' requirement is completely different from those of the STS or the Mach Trim.

Wow, I had nothing right lmao. So I’ve pilots talk about any unwanted pitches, mean you hit the stab trim cut out switches? So that’s why I assumed what I did. So before the Max, since the NG had an AOA, What would it relay info to?

@Firefighter1 As far as I know, it's only used for stall warning and cockpit display.


text:
Can a 737 be on final at 160 knots, and also touchdown at 160 knots?
Would the 737, going 160 knots, float a bit more before touching down?

You fly final approach at the "reference speed" (Vref) for the flap setting which varies with weight.  You will normally set up the reference speed for full flap on the speed bug on the speed tape on the primary flight displays, and on final while the autopilot flies the glide slope you will modulate thrust to hold the approach speed (or let the autothrottle do it if equipped).
For a given landing weight, the reference speed for full flap is worked out from the "speed book" or "speed cards" which the crew usually has handy on the center console.  The airplane on short final with landing flap will be going at Vref, which is usually well below 160kt, typically between 110 and 150 kt for different airliners at different weights.
As you cross the end of the runway and pass through 50 ft, you should be at Vref.  As you get close to the surface and start to flare to land, you reduce thrust to idle at some point (the technique varies with the airplane) and as you hold it off the speed will bleed off as you trade inertial energy for reduced sink rate in getting a gentle contact with the runway.  In most jets you want to be at idle before touchdown, with exceptions.
So in the interval from when you started to flare and reduce thrust, and the actual touchdown occurs, you are coasting along, settling gently as you pitch up to trade inertial energy for reduced sink, with the speed bleeding off as you do so. At touchdown you will be below Vref, perhaps 10-20 kt.  So if a 737s Vref for a given landing weight was some number like (wild guess) 135 kt, by the time it makes ground contact, depending on how much effort the pilot put into holding it off trying to get a gentle touchdown, it'll be doing perhaps 125-130 kt.
In any case, touchdown speed is always below the approach reference speed, unless the pilot just flies into the runway like a carrier landing, which will get lots of people excited.
160 kt on short final is pretty fast for most jets, but you could be going that fast if the flaps had failed while retracted or at an intermediate position and you had to use a higher approach speed (on the CRJ200, the reference speed with flaps failed fully retracted, which they do from time to time, is about 165 kt +/-).  Same thing though; during the landing itself, the touchdown speed should be less than the final approach speed due to the energy conversion process of the flare and touchdown.
If your reference speed was 140 kt and you crossed the threshold at 160, yes you would float quite a bit if you used normal landing technique. You would also get sent for remedial training.  If you tried to drive it on at that excessive speed, there is a good chance you would have nose wheel contact first (called a wheelbarrow), which would get people even more excited.

Yes, if spoilers are deployed.  True to their name, spoilers reduce the amount of lift a wing produces at a given speed and AOA. They also help slow the plane down by adding drag.
Although "ground effect" will contribute to "floating", flaps, slats, and spoilers, essentially speed brakes, all help slow the plane down enough to land.

Hmmm... langing with spoilers deployed hardly is SOP, but as an answer I guess this would be viable. As a landing technique, not so much.

It is theoretically possible, and spoilers, like flaps, could be applied in increments.  They basicly make the wing "smaller", so you need either more speed or more AOA for the same amount of lift, as well as adding drag. John K's description of a hapless airliner "wheelbarrowing" in at excessively low AOA would not be a good day.  In smaller aircraft this leads to "porpoising", in a larger one it could completely collapse the nose gear.


text:
Does “trimmed for current speed” mean that the plane will do whatever to maintain the current airspeed?
If I understand right, when you are trimmed for current speed, let’s say 290 KTAS, does that mean if airspeed increases to 295, the plane will pitch up to slow it back down? And vice versa for the speed falling below 290. The plane would essentially dive on it, to get the speed back? Or am I way off?

Pilots often don't really learn how to use trim, because they don't really grasp that pitch is fundamentally a speed control and the trim wheel is basically hands-free speed control dial.

@JohnK, this is also not quite correct and is somewhat of a misconception. 'Fundamentally' pitch is pitch. In short-period motion (which is often no less important) it is more related to AoA than to speed. Only in steady state your view (approaches to be) correct.

The pitch trim setting is based on the angle of attack.  The angle of attack is affected by indicated airspeed, center of gravity and any unbalanced forces.
In a steady state aircraft where the airplanes forces are all balanced then we are left with airspeed and center of gravity.  Under normal operations where fuel is not being transferred from an aft tank to the wing tanks or vice versa, center of gravity for a certain period of time remains relatively constant.  This leaves airspeed as the primary change in angle of attack.
Your analysis would be correct.  If you are trimmed for 290 KIAS (not KTAS like you stated) and you increase thrust the following sequence will occur.

Airplane will begin to accelerate
The lift formula comes into play and additional lift is created on the wings causing the airplane to pitch up and start to climb.
The airplane experiences an increase in induced drag which opposed thrust.
The airplane will now begin to decelerate while still climbing.
The decreased airspeed causes lift and induced drag to decrease.
The aircraft will pitch down and start to descend.
Aircraft will begin to accelerate as it descends.
The cycle continues until the phugoid oscillation dissipates (provided the airplane demonstrates positive dynamic stability)
In the end, the airplane will be at a higher pitch attitude (in relation to the horizon), climbing, and at 290 KIAS.

The same type of analysis can be accomplished if airspeed is reduced.  The airplane, at the end of the phugoid oscillation, will be at a lower pitch attitude and the same airspeed.

The sequence is wrong. First, think: in the end, where does the excess added thrust go? Second, why does additional lift due to speed cause the airplane to pitch up?

Yeah, I think @Zeus is right here. If the airflow at the beginning doesn't produce a pitching moment, then after the airspeed increases, it still won't produce a pitching moment (as long as the angle of attack doesn't change—which it won't right away, if the aircraft is in level flight before and after the power increase.) Instead, the increased speed will cause the wings to produce more lift, which will cause the aircraft to climb, and that will cause the aircraft to pitch up.

Also, you haven't mentioned the fact that while the aircraft is climbing, lift (not just drag) will cause it to slow down, and while it's descending, lift (not just a reduction in drag) will cause it to speed up. I don't know which of the two forces has a greater contribution, but my guess is it's lift. In particular, even a drag-free aircraft would still undergo phugoid cycles.

@wbeard52, you are missing the energy trade between kinetic and potential in the behavior. Without that, no phugoid (but you still have short period).

@Zeus is right, if you just increase thrust without changing anything else you will end up at 290 Knots as phugoid dies down, but you will be climbing. Climb angle will depend on how much thrust you added.

@kolom  You are right.  I made an assumption that when an aircraft pitches up that was understood as also climbing.

Additional lift on the wing caused by acceleration alone does not cause the plane to pitch up. It causes it to climb, but does not in itself change the pitch angle. Most planes have horizontal stabilizers with negative lift. When the plane accelerates, the wing gains positive lift, and the horizontal stabilizer gains negative lift - this causes the plane to pitch up.

No @Greg, when the wing gains positive lift, it also gains moment that the tail downforce is compensating, so you need more downforce just to stay balanced. With airspeed changes, this all happens in exact proportion. If you fix a model in a wind tunel on a hinge, it will stay balanced at different speeds. It is acceleration upwards (due to increased lift) that changes the AoA in different proportion on the wing and the tail, and hence causes pitch change.

No Zeus, it’s a combination of what I described above and the relatIon ship between the cg and the line of thrust and center of lift. See pages 5-15 through 5-17 of the handbook of aero knowledge. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/07_phak_ch5.pdf

@Greg, 'line of thrust' has nothing to do here. The phenomenon exists even for gliders. If we involve thrust, it can both increase and reduce stability, but if we assume that thrust doesn't change with speed, it will have no effect except for adding a bias. The fact of the matter is that changing speed alone does not change the moment balance (of the lift) - but in practice this can only be seen in a wind tunnel.

As I understand this answer, it is suggesting that the phugoid oscillation is driven primarily by changes in angle of attack (note the specific reference to induced drag rather than total drag).  Why would you say that climbing causes any increase in drag at all, either induced drag or total drag?

My understanding is that to a first approximation, angle-of-attack can be viewed as being constant during a pitch phugoid oscillation-- fundamentally what is going is that airspeed, and therefore lift, is "out of synch" with what would be required to maintain flight at the given angle-of-attack and the given instantaneous direction of the flight path.  (And also, the direction of the weight vector relative to the flight path is constantly changing.)

Yes, in practice the airplane will do "whatever" (as you describe) to maintain its "trim speed". The airplanes are, in fact, reqired to do this. So if you are a pilot and practical considerations is all that matters, this is enough to know. (Perhaps though, you will also be interested in the dynamics of this process and how to control it better).
However, if you are interested to know why the airplanes do this and understand the fundamentals, read on.
First thing to note is what the trim actually does. There are different mechanisms, from aerodynamic tabs to direct stick force control, but what they all do is setting the elevator (or the whole stabiliser) at a certain angle with respect to the wing when no force is applied to the controls.
Now it is ieasy to show that the airplane is trimmed not for speed but for an angle of attack (AoA).

In a level steady flight, we have both the force and moment balance: $L + L_t + mg = 0, Lx_{CP} + Lx_{L_t} = 0$ ($x$ is the arm with respect to centre of gravity CG; all forces are vectors). If we simply change speed, none of the points will move (until we get close to the speed of sound). All the aerodynamic forces will change proportionally (as square of speed). It follows that the moment remains balanced. This can be verified in a wind tunnel: a hinged model will remain at the same angle if we change airspeed.(*)
If we change the stabiliser angle ('trim' it), only $L_t$ will initially change. This will change the pitch balance, and the airplane will start to pitch - until it finds another balance angle. Here, CP will move and $L$ will change, but in the end $Lx_{CP}$ will again be equal in magnitude to $Lx_{L_t}$. This will be the new 'trimmed' angle of attack.
On all 'normal' aircraft (that is, human-sized), this AoA change will happen a lot quicker than speed or altitude change. The aicraft will settle on the new AoA, and only then the rest will happen. Namely, with new lift (at the new AoA) comes vertical acceleration, and also new drag. This all combines into changed trajectory and speed.
As you see, the traditional pilots' belief that trim sets speed isn't entirely correct. A pre-requisite to stabilising on a new speed is that the airplane stabilises on a new AoA. This is why by 'static longitudinal stability' engineers understand AoA stability rather than speed stability. If the airplane is unstable in AoA, it will not live to see 'speed stability', or anything else for that matter.
But what actually happens to a nicely trimmed airplane when only speed changes? Well, by definition, initially it does not involve AoA change, and so the process is relatively slow. As I explained above, the moment balance does not change, so there is no tendency to pitch. However, changed lift will cause vertical acceleration (let's say, for simplicity of explanation, that speed and thus lift increased). This causes two consequences:

The energy for moving up must come from somewhere, and the airplane will start to decelerate - unless it increases thrust.
Vertical speed will change AoA: now we have a vertical airspeed component. This component is equal for both the wing and the tail, so the absolute AoA change is equal (say, -2°). Now the usual thing will happen: since we haven't re-trimmed, the airplane will tend to restore its trimmed AoA, which means pitching up by the same amount (+2°). And this will happen rather quickly: the airplane will 'track' AoA according to the current vertical (and horizontal) speed.

The ability to track (stabilise) AoA means that the equilibrium can only be restored if speed is restored to its previous value. This will eventually happen, but this will happen slowly because it involves exchange of potential and kinetic energy. There will likely be an oscillatory process (known as phugoid motion), where the initial rise and pitch up sets the airplane for climb, it loses speed more than it should, loses lift, starts to descend, and the process reverses. But once again, AoA stability is the driving factor here.
It is possible to make an aircraft which will be statically stable in AoA but unstable in speed. For example, by having strong positive dependency of thrust on speed (turbojet engines are like that at certain speed ranges). But because speed changes are slow, such aircraft can still be flown by a human - unlike AoA-unstable aircraft.

(*) Strictrly speaking, if we increase speed substantially (and instantly), AoA will reduce a bit. But AoA stability, as explained below, will quickly restore AoA, and we'll be left with just changed forces.

You wrote

If I understand right, when you are trimmed for current speed, let’s say 290 KTAS, does that mean if airspeed increases to 295, the plane will pitch up to slow it back down? And vice versa for the speed falling below 290.

You are describing the phugoid mode for longitudinal dynamics. The key is that it trades kinetic and potential energy back and forth.
The phugoid behavior occurs at approximately a constant angle of attack, or coefficient of lift $c_{L}$. The lift is just a function of speed.
Faster = more lift, and when the lift exceeds weight, the aircraft pulls up. As it goes up, it trades kinetic for potential and slows down. Slower = less lift, and when the lift is less than the weight, the plane arcs over the top and heads back down, trading potential for kinetic.
There are other physics going on, but that is the core of it.
If you were to try this in a huge space station with no gravity, you wouldn't see the phugoid. The energy trade is a key part of the behavior. You would only see the short period.

Yes airplanes trim to a speed.  The airplane is "in trim" when the tail downforce balances the nose down pitching moment about the airplane's neutral point and it is not accelerating or decelerating.  Left on its own (no control input), it will pitch to seek its trim speed when displaced from that speed for whatever reason.  If actual speed is below trim speed, the airplane will pitch down because tail downforce is below the "balance value" (and vice versa if it's above).
So if you are below trim speed for some reason, if you just leave the airplane to itself, no hands on the controls, the airplane starts to dive due to too little downforce, and accelerate. It shoots past its original trim speed in the dive, where it now has too much elevator downforce, which makes it start to pitch back up.  It will then overshoot going the other way, ending up in a decellerating climb, but a bit less than the initial time.  It ends up too slow and starts to dive again on its hunt.  Each of these Phugoid oscillations gets less and less and eventually the airplane finds its original trim speed again, more or less.
All this is happening with your hands on your lap.  You can short circuit the whole process with timely elevator inputs if you want.
This is "static longitudinal stability".  FAR 25.173 requires a minimum level of this "speed seeking" in response to displacement from trim speed and specifies a minimum ability to return to trim speed once displaced (within 7.5% in cruise flight).  The airplane will be trimmed for some speed, say 250kt, then slowed down or sped up without any trim change, then left by itself with no input, has to be able to find its way back to trim speed within x knots tolerance, preferably with a relatively small number of Phugoid oscillations as it dives and climbs on its hunt for trim speed.  Too slow, it should dive by itself (gently), too fast, it should climb.
In cert testing they will push the nose over with an elevator input, forcing it to dive and accelerate to say trim speed +50kt (or climb and decelerate to go the other way), then let go and watch what happens.  It should slowly climb and dive, climb and dive, less and less each cycle, until it settles down to hopefully a few knots from the original speed with nicely damped, diminishing oscillations as it does so.
Trim speed is also affected by changes to the balance of pitching moments and forces, like adding or reducing thrust where the thrust line makes the nose pitch up or down. But for this case we are talking about steady state conditions.

No. Like StephenS (please see my comment to his answer), you are missing a vital link between speed and picth changes. "If you are below trim speed", you will have less downforce, but you will also have less wing lift and moment to compensate with this downforce. The pitch balance doesn't change at this point. Also, "static longitudinal stability" is a different thing; technically, it relates to response to AoA disturbances. It is related, in turn, to speed stability you are talking about, but they are not the same thing. (This relation is the actual answer to the question).

Yes the wing's lift decreases along with the tail's downforce but the net result is that the downforce is insufficient and the nose down pitching moment about the neutral point is able to overcome down force and the nose drops.  FAR 125.73 which describes the speed seeking behaviour requirements I described is TITLED "static longitudinal stability".  But, whatever; go argue with the FAA.  Dynamic Stability describes how well the system is damped when displacements occur per FAR 23.181 https://www.risingup.com/fars/info/part23-181-FAR.shtml

@Zeus I don't know what the fuss is. Static longitudinal stability = speed stability.

@JZYL, not quite. This is a secondary effect. I had to write an answer to explain.

@Zeus You're redefining a term that has been defined already. In both industry (airplane certification) and academia (see references on Etkins), static speed stability and static longitudinal stability are exactly equivalent. Your answer is correct in that AOA stability and speed stability are not exactly the same. But the term, static long stab, is used exclusively by any certification agency to mean speed stability.

@JZYL, I studied (and worked) in academia for quite a few years, and we primarily understood AoA stability by this term. Technically, both fit the term (both are longitudinal and both are static), and the context (short-period vs long-period) determines which one is meant. Pilots, however, hardly understand short-period (this stability is 'a given' to them), so for them it's nearly always speed stability. Certification follows the same path: they specify what is important for the pilot, after all; and it is easier to measure.

Actually most pilots tend to think, incorrectly in my opinion, of stability and trim in terms of pitch attitude, helped along by the NU ND marks on most trim wheels.  For practical use, I consider it better to think of the trim wheel as the "hands off" speed control dial, and when I work the trim it's usually because I'm speeding up or slowing and only use it in the pitch control sense when fine tuning for level flight.  Pilots who learn in gliders first get this a lot better than those who don't because the glider doesn't have power to complicate the concepts.


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Is it possible to stall a plane so badly that the nose refuses to go down due to lack of airspeed?
So basically, a stall from a high AoA to the point that the entire plane just falls with the rear end pointing straight down?
Because if wind is no longer passing over the wings, then this includes the elevator, and the elevator can’t deflect the wind with no forward speed?

If your aircraft is aerodynamically stable, the slightest perturbation from this rear-first flight will tend to turn your aircraft front first

If you drop a jet fighter from a hovering helicopter, this is pretty much what happens. (Or a toy jet fighter from your hand)

If you drop your jet rear-end first, there will be airflow over the wings, just in the opposite direction.

Do you really mean "rear end pointing straight down", or just pointed downwards in general?  Many of the answers below reference a condition where the aircraft is pitched up, leading to insufficient speed to provide sufficient control authority to change the attitude of the aircraft.  This condition is not referencing an attitude with 90 degrees of pitch (ie tail pointing straight down), but I think it meets the intent of your question.

Yes it's called a Deep Stall, and is mostly a problem with T Tail aircraft, especially jets with Supercritical Airfoil wings (like the CRJ Regional Jet line).
Such wings stall from the leading edge and the stall's flow separation spreads rapidly and completely across the entire wing all at once, so there is very little residual nose down pitching moment.  Plus the T tail ends up in a spot where it's in the wake of the flow from the wing, and gets blanked out, and loses its ability to pitch the nose over with positive lift since it's sitting there in the turbulent wake of the wing.
So the plane just mushes downward in an unrecoverable, stabilized, well, mush, and will pancake into the ground like that.  A CRJ200 test aircraft was lost in development testing when it got into a deep stall and IIRC, the stall/spin recovery parachute in the tail failed to deploy (or it didn't have one at the time; I forget which).
Such airplanes require, in addition to the usual stick shaker, a stick pusher to force the nose over, by shoving the control column for you if you do nothing while the shaker is going off, before the natural stall can occur, since the natural stall can be unrecoverable.  Generally if an airplane has a stick pusher system it means it has a deep stall mode.

Some T-tail gliders are also affected. It's particularly bad because gliders are often normally flown in near-stall conditions. Still, as they are agile enough to spin (unlike jetliners), you can usually go from a deep stall into a spin, and recover from there (but you lose a lot more altitude with that than in case of a simple horizontal stall recovery)

You mean it needs an MCAS?? ;-)

@vsz: No, that would be a regular stall. Deep stall is a very different affair.

Why would a supercritical airfoil facilitate super stalls? The first case had conventional airfoils, as did most other cases. Once the wing is fully stalled, the details of the upper wing contour don't count anymore. Tail engines which shift the cg (and thus the wing) closer to the tail and a T-tail are mostly to blame.

@PeterKämpf : I wouldn't say it's a regular stall if the elevators aren't working at all in the stall. The only way in that particular case is to struggle with the ailerons in the hope one of your wings drops and you start entering into a spin. It takes time and you lose a lot of altitude. Compare it to a regular stall from which you can easily recover in a typical glider with a simple push of the stick while not losing much more than 50 meters of altitude. (and the instructors did call it deep stall)

@vsz: Now I wonder about several things: Which glider goes into a deep stall and then in a spin? Which jetliner is unable to spin (given an enough back cg position)? There was a NASA-modified Schweitzer 1-36 which could do deep stalls - do you mean that one? And can you guess why it had to be modified for deep stall research?

@PeterKämpf : I didn't say it goes into a spin from a deep stall automatically. And I don't know everything about jetliners but I've never seen or heard examples of them in a spin.

The SC wings suffer from LE stall initiation due to the laminar bubble that forms at the somewhat squared off LE profile.  When the flow reattaching behind the bubble lets go at any one spot it triggers a cascade effect and the flow separates from the LE along the entire span all at once.  The airplane finds itself in a deep stall mode immediately.

What you describe is a tailslide, as another answer has noted -- but there is a condition in which the wing is stalled and the normal recovery method (apply down elevator and wait for the nose to drop and airspeed to build) can't be used.
It's called a "deep stall" and is only a problem with certain layouts of flying surfaces.  One of the best known is a T-tail in which the spacing between wing and horizontal tail is such that it's possible for the turbulent wash from the stalled wing to completely blanket the stabilizer and elevators, making it impossible for those surfaces to overcome the drag pushing the nose up.  This can make a deep stall unrecoverable in certain T-tail aircraft (the F-104 was infamous for this, and it also affects some sailplanes).
Worthy of note is that deep stalls (commonly called a "falling leaf") were a common maneuver in the days of fabric covered biplanes; many of these had enough elevator authority to hold the stall, and enough rudder to keep the stall "straight ahead" rather than letting it turn into a spin.  They were used as an alternative to a slip, to dump altitude without letting airspeed increase excessively.  Most monoplane designs (that I'm familiar with) don't have enough pitch authority to hold a deep stall (part of making aircraft spin resistant is making them stall gently), so the maneuver has fallen out of general familiarity.
The difference between a controlled deep stall and an unrecoverable one is loss of pitch authority due to blanketing of the horizontal tail.

The falling leaf is not a deep stall https://en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_stall.  A deep stall is by definition unrecoverable.  You can fly a lot airplanes with full aft stick and the thing shuddering and shaking and bobbing with bits of rudder to keep it from rolling off.  I do it in mine from time to time.  The outer wing is still unstalled and even if the tail gets blanked off there is still significant pitching moment in the wing to drive the nose over.  It's only a deep stall when there is no tail authority and there is minimal or no pitching moment present.

The answer is a little misleading at the beginning, but he does not actually say that a tailslide is a deep stall. I agree, a “falling leaf” stall is not a deep stall.

@JohnK The definition must have changed.  I learned (long ago) that a "falling leaf" was in fact an artificially maintained deep stall, just as the "dethermalizer" on a free flight model induces a deep stall, but (if R/C DT) the aircraft will recover if the 30-45 degree decalage is restored to normal value.

Yes you are "deeply stalled" but the tail is still generating downforce to oppose the pitching moment (remember that during the stall the CP moves aft, increasing pitching moment putting more downforce demand on the tail) there is kind of an oscillation between more or less stalled as the tail loads and unloads.   However if letting go of the stick results in recovery, it's not THAT deep stall, where the tail is not able to generate any pitching moment either up or down and there is little pitching moment from the wing itself.

The characteristic of a deep stall is a second pitch stability point at high angle of attack. Yes, this can be achieved with a huge negative deflection of the whole horizontal tail surface (elevator control isn't sufficient) as done in free-flight models. Now the tail will double as a speed brake and lose all pitch authority, so the aircraft drops out of the sky. No manned glider that I am aware of is capable of this kind of descent.

Yes.  Moving CG farther and farther back will eventually cause the plane to be directionally stable falling backwards.  Abusing rearward CG limits is contributory to this condition.
Secondly, poor design of horizontal stabilizer, particularly, lacking sufficient "weathervaning" area, will cause an aircraft to be more susceptible to the unrecoverable "deep stall".  With models, this is tested by holding the plane horizontal to the ground and releasing it with no forward motion.  The relative wind, being 90 degrees to the wing and tail, means they are both stalled, but the pitch torque from the DRAG on the horizontal stabilizer, the rear fuselage and the trailing edge of the wing should flip the nose down and unstall the plane.
Higher aspect wings and/or a shorter fuselage requires greater tail area for the same pitch torque, AND, a larger weight to surface area ratio (bigger plane) also requires a greater tail/wing area ratio.
Placement of the all important horizontal stabilizer can also affect its performance.  If it is in the "shadow" of the wing, as with T tails, a very high angle of attack can limit its ability to create pitch down torque.  Downwash from the wing can also affect a "low" Hstab.
Lengthening the fuselage is a remedy for not only wing airflow effects, but also increases the pitch torquing lever arm of the aircraft, enabling the same size Hstab to be more effective.
Thrust angling is also a key factor to assisting in pitch down torque.  Many aircraft have their thrust line angled down a few degrees, which helps control pitch up tendency as the plane accelerates.
Choice of sound and proven design is important, as well as keeping CG within limits.

As a comment on abusing C of G, in hang gliding it's not uncommon for pilots to misjudge and flare with too much airspeed, with the inevitable result of zooming as much as 15-20ft up. The hang glider flare is essentially pushing your C of G all the way back. It's essential to hold the flare if this happens, so the glider tailslides down stably - you'll come down hard, but it'll usually be OK. If you panic and let it go though, the glider immediately goes nose down to try to recover the stall, which typically would take around 100ft or so. At 15-20ft off the deck, this is not a good thing.

If a conventionally laid out aircraft is flown into a tail slide, it's unlikely that it could maintain that attitude for long. The tail has low mass relative to the rest of the aircraft and a substantial moment between the tail surfaces and aircraft center of mass. As negative airspeed (falling down tail first) builds, aerodynamic forces on the tail will cause it to flip around. Angular momentum will continue the rotation until there is enough opposing aerodynamic force to stop it. While a deep stall may be unrecoverable, a tail slide should be recoverable, given a sturdy airframe, enough altitude and the correct control inputs properly timed.
Hypothetically:
Given an aircraft of conventional layout, if it was released at zero airspeed and level attitude, the greater surface area of the tail as well as the greater surface area of the wing aft of its center of gravity will create a force that will rotate it nose down. What distinguishes this sort of scenario from a deep stall is that the condition is unstable and robustly forcing the airframe towards a normally-oriented airflow.

No, it is not possible that the nose will “refuse to go down” with “the rear end pointed straight down”.
If the aircraft is going down tail first, then there IS airflow over the wings. It may be briefly in the wrong direction, but the center of gravity and placement of the wings will soon have it pointed in the correct direction. With the nose pointed down again, the wings may still be stalled, but there will be airflow over the wings, and the stall will be recoverable with the correct control inputs.

I have a friend who experienced an actual natural stall on a CRJ200, when the flow was tripped by extruded sealant on one leading edge, and the wing stalled before the normal pusher firing point during a test.  What saved them was I believe the fact that only one side let go and it rolled off knife edge and fell off that way more or less as a lawn dart, so that the vertical fin would get the nose pointed back down again and he was able to roll it straight and recover from the dive.  Caused by releasing the a/c before the LE gap sealant had fully cured. That's all it took.


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What is the suggested position for a canard?
In a canard aircraft, what is the suggested position of the canard, with respect to the wing positioning? Considering the effect of down wash of the canard on the wing, what is the suggested position for a canard? Mid canard? High canard? Low canard?

There are two general configurations for canard wings, the long coupled and the short coupled canards. From Torenbeek:

Long coupled canard. This configuration is designed for minimal drag in cruise. The canard wing is placed far forward of the main wing in order to reduce interference drag. Vertical position is higher than the main wing to reduce adverse effects at high angle of attack.
Short coupled canard. The canard wing is again higher than the main wing: the interference is deliberately used to increase maximum lift (at the cost of high drag), achieved:

at large angles of attack;
on surfaces of low aspect ratio with highly swept back leading edges.

Quoting from Torenbeek:

In short, the canard layout appears to be suitable for transonic or supersonic and highly maneuverable aircraft, in the latter case if sufficient thrust reserve is available


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Do we artificially add a smell to jet fuel?
Many dangerous liquids have odorants added so that you can know if something dangerous is leaking.  Is anything added to jet fuel (such as Jet A-1) to give it a distinctive smell?

Kerosene (main component of Jet A-1) already has a quite unpleasant odor, so why would it be necessary?

@Bianfable How can it be unpleasant when it smells like flying ?

No kerosene stinks pretty good pretty good on its own so it doesn't need any stinkifiers added.

Obligatory quote: "I love the smell of jet fuel in the morning.  It smells like victory."

@Quentin H: Because to me, Jet A smells like COMMERCIAL flying, which to me is a very unpleasant experience.  Avgas is different :-)

@jamesqf: Don't sniff avgas, it contains lead.

@Sean: One can hardly avoid smelling avgas (or ordinary automotive gasoline, FTM).  There's a trivial amount of lead in the vapor that's enough to produce a detectable smell.

Aviation fuels are all hydrocarbons that have fairly strong natural odors, so no additives are necessary to give them a noticeable smell. Jet-A1 is refined kerosene, which has a distinctive smell which some find unpleasant and others not. Aviation Gasoline (AVGAS) has many "aromatic" hydrocarbons to boost octane which are called aromatic because of their distinctive odors.
So you can smell a fuel spill or leak pretty easily, for at least AVGAS I know this from experience. I once had a fuel leak which filled the cockpit with avgas fumes, I got a lungful before I managed to ventilate the cockpit and it was very unpleasant!

Technically "aromatic" refers to the stability of the molecular structure but it does apply to the odor here as well.

That's true now, but when they were first being studied they gave them that name because many had relatively pleasing odors @fooot

Avgas also contains considerable quantities of tetraethyllead, which has its own very distinctive odour (fortunately not something I know from personal experience, as TEL is also extremely toxic).

It's toxic to ingest @Sean, but getting whiff of it isn't harmful. When you test fuel for water content as part of a pre-flight check one of the steps is to sniff it to make sure it's not all water.


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How does the noise of a 2-to-4-seat helicopter compare with that of a similarly-sized fixed-wing aircraft?
We are considering allowing helicopters on our private airstrip (2600 ft strip). Noise on the ground (and approach) is the concern from the community.
Does anyone know approximate dB ratings for a typical 2-4 seat helicopter vs. a 2-4 seat fixed-wing propeller aircraft? I know there is no exact comparison, but just an approximation would be helpful.

This question is too broad as asked.  Noise signatures vary. A helicopter could be a robinson or an MD 500 or a Long Ranger.

I found one study comparing helicopter and airplane noise levels and signatures:
Aircraft source noise measurement studies summary of measurements...
Unfortunately this study does not provide conclusions or any  abbreviated comparison in plain language (or I just failed to notice them). It does, however, provide a near endless array of tables to shuffle though and compare.
According to the study there are similarities and differences in noise signatures between helicopters and airplanes, especially depending on the phase of flight and this hardly comes as a surprise.  In real life the biggest problem arises form the differences in how people react to sounds. Some of us might not care much about even the loudest jet noise (I, for example live a couple of miles form an AFB, and I don't mind), and some experience the slightest noise as a terrible nuisance.
What I would recommend, based on my very limited (but still academic) studies on acoustics, is to conduct a series of test flights and measurements to:

verify that legal (if any apply) decibel limits are not exceeded
give you measurement data for comparison and later reference
and, most importantly, to give you and the area residents an idea of the noise related to helicopter operation.

It should be noted that noise charts, such as those presented in the study above, are not easy to interpret. Decibel is a rather tricky unit, and does not necessarily correlate with the way one experiences noise. In comparison, for example, helicopters might be louder in decibels in some stages of flight than same size airplanes, but the noise might not be experienced any more disturbing (or pleasing, depending on how one is inclined)
Human ears are by no means precision instruments, and actually neither are high end decibel meters: their accuracy is typically somewhere in the range of 1 decibel, and considering it equates a difference of 26% in sound energy, and 3 decibels would be doubling or halving the sound energy, it's not very precise... To make things more complicated, an increase of 1 dB is just barely noticeable by human ear (or brain). 3 dB is perceived as about a quarter louder, and 10 dB feels twice as loud.
So: there's not much to distill from decibel charts or measurements unless the differences are huge across the whole frequency range (more than 10 dB), and even then it's still somewhat ambivalent... It's best to get real life experiences when it comes to sounds.

I notice your question has been unanswered for some time, now. I avoided an answer due to not having any experience or data comparing the two (As far as amplitude of noise). I can say that the frequencies of the noise are different. Engine noise /freq are roughly the same for both FW and RW. FW props produce a blade pass freq of #blades x rpm and for the size of aircraft, props and tail rotors are similar in frequency. The big difference is helicopter main rotor noise is much lower in frequency. The fact that the main rotor is producing the lift AND thrust is another thing to consider.The other thing is that helicopters operate at relatively higher power during approach and landing. You might get a phone sound meter app and go to an airport and compare. hope this helps.